The present invention relates to a method and a system for acquiring motional information of an object in only one scan in a magnetic resonance imaging (hereinafter referred to as "MRI").
Conventionally, the phase difference of spin echo signals is utilized to obtain a blood flow rate in MRI. For instance, images of an object in systole and diastole periods of a heart are acquired in synchronism with a cardiogram signal, and in view of the fact that blood flow rates in these periods differ from each other, these images are subtracted from each other to acquire the blood flow rate. In this case, to obtain the images for the heart's systole and diastole periods, it detects an echo signal from an excitation caused by application of a 90.degree. pulse to the object. Therefore, the scan is performed twice to obtain the two images. That is, it performs twice the scan for obtaining an echo signal by applying a 90.degree. pulse and then a 180.degree. pulse to the object.
Since the conventional method performs the scan at least twice, it is difficult to detect the blood flow rate in a short period of time.
Further, as explained below, it is difficult to detect the blood flow rate by one the scan. For example, the spin phase .phi. of an echo signal, which is acquired after a time T.sub.E /2 from a point at which a 180.degree. pulse has been applied after a time T.sub.E /2 from application of the 90.degree. pulse, i.e., after a time T.sub.E from application of the 90.degree. pulse, is taken by the following equation. EQU .phi.=-.gamma.GvT.sub.E.sup.2 /4+.phi.system (1)
where G is the intensity of the gradient field, .gamma. is the magnetogyric ratio, v is the velocity of an object and .phi.system is a systematic phase error of an MRI apparatus. In equation (1), .phi., G, .gamma. and T.sub.E are known. However, .phi.system is unknown, the accurate velocity v cannot be obtained without considering the term .phi.system.
Therefore, there is a demand for a method for acquiring motional information of an object by only one scan without any influence of the systematic phase error of the MRI apparatus.